Segment Information (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Rental and other revenues	$ 181,388	$ 179,417	$ 178,792	$ 180,438	$ 175,861	$ 180,185	$ 177,283	$ 169,408	$ 720,035	$ 702,737	$ 665,634
Total net operating income									477,620	465,849	434,549
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Depreciation and amortization									(229,955)	(227,832)	(220,140)
Impairments of real estate assets									(423)	(1,445)	0
General and administrative expenses									(40,006)	(39,648)	(38,153)
Interest expense									(71,422)	(69,105)	(76,648)
Other income									1,940	2,283	2,338
Income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates									137,754	130,102	101,946
Total Assets	4,675,009				4,623,791				4,675,009	4,623,791
Office Total Segment [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									707,158	689,788	651,511
Total net operating income									468,213	456,628	424,731
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	4,517,153				4,391,387				4,517,153	4,391,387
Office Atlanta, GA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									141,337	140,323	134,601
Total net operating income									87,503	89,575	84,733
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	1,047,850				1,049,100				1,047,850	1,049,100
Office Greensboro, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									22,322	21,453	20,522
Total net operating income									14,275	13,612	12,781
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	118,611				134,194				118,611	134,194
Office Memphis, TN [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									40,230	45,430	48,251
Total net operating income									25,659	28,128	30,038
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	213,276				218,088				213,276	218,088
Office Nashville, TN [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									121,836	111,506	95,912
Total net operating income									88,554	81,204	68,678
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	937,732				806,725				937,732	806,725
Office Orlando, FL [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									53,771	51,236	46,260
Total net operating income									32,841	30,526	26,525
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	306,370				306,970				306,370	306,970
Office Pittsburgh, PA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									61,177	59,103	58,789
Total net operating income									36,233	34,784	34,175
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	329,918				334,136				329,918	334,136
Office Raleigh, NC [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									118,352	119,254	112,958
Total net operating income									86,053	86,475	80,803
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	792,464				762,331				792,464	762,331
Office Richmond, VA [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									45,729	43,959	44,315
Total net operating income									31,276	29,946	30,505
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	248,669				229,468				248,669	229,468
Office Tampa, FL [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									102,404	97,524	89,903
Total net operating income									65,819	62,378	56,493
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	522,263				550,375				522,263	550,375
Other Segment [Member]
Segment Reporting Information [Line Items]
Rental and other revenues									12,877	12,949	14,123
Total net operating income									9,407	9,221	$ 9,818
Reconciliation to income from continuing operations before disposition of investment properties and activity in unconsolidated affiliates:
Total Assets	$ 157,856				$ 232,404				$ 157,856	$ 232,404